Have you just tried using a Play module and in that Play module specifying the WAV file and then also placing some text (or the $RV if it is defined) in the "Text to speak:" text box and see if both are played correctly? You will need to ensure the TTS engine generated sound and the WAV file formats match.

Otherwise you may need to use two separate Play modules one after another - with the first one playing the WAV file and the second one playing the TTS (linking them using a "Timeout 0" path and specifying 0 replays in the WAV module)